Project Summary Postpartum depression is one of the most common complications of childbirth. Untreated postpartum depression can have substantial adverse effects on the well-being of the mother and child, negatively impacting child cog- nitive, behavioral, and emotional development with lasting consequences. The underlying pathophysiology of postpartum depression is largely unknown, creating an obstacle to developing improved treatments. The overall goal of this study is to determine the relationships between GABAergic and glutamatergic neuroactive steroids, cortical GABA and GLU concentrations and default mode network resting-state functional connectivity in women with postpartum depression versus healthy postpartum and non-postpartum women. Our central hypothesis is that high sustained amplitude neuroactive steroid exposure then withdrawal during the pregnancy and the post- partum period versus the low brief amplitude neuroactive steroid exposure/withdrawal of the menstrual cycle, is associated with differential cortical GABA and GLU concentrations within the default mode network and their correlation to network resting state functional connectivity, with further differences between women with and without postpartum depression. This study is a prospective observational study of 54 healthy peripartum comparison women and 54 peripartum women with postpartum depression in comparison to a cross-sectional examination of 54 healthy women in the follicular stage of the menstrual cycle. Peripartum mood data and blood specimens will be ob- tained at 2 antepartum and 2 postpartum study visits. Healthy postpartum comparison women and women with post- partum depression will undergo a single functional MRI and magnetic resonance spectroscopy scan in the postpartum period while healthy women in the follicular phase of the menstrual cycle will complete a single study visit including mood, blood and neuroimaging measures. The study will examine differences in default mode resting state functional connectivity across groups (Aim 1), differences in GABA and glutamate concentrations across groups (Aim 2), corre- lations between spectroscopy and resting-state functional connectivity (Aim 3) and explore correlations of spectros- copy, resting-state functional connectivity, neuroactive steroids and depression in postpartum depression (Exploratory Aim 1). Finally, half of the women with postpartum depression will complete a second imaging session after successful postpartum depression treatment (Exploratory Aim 2) to determine GABA, glutamate and resting-state functional con- nectivity changes associated with treatment. This data can then serve as an objective index against which to predict response to current pharmacotherapies, identify new postpartum-specific therapeutic targets and measure po- tential efficacy of newly developed treatments.